The present invention relates to dielectric ceramic compositions adapted for use at microwave which is especially, but not exclusively, useful for devices or equipment for communication or broadcasting.
In recent years, there have been used dielectric materials having a high dielectric constant, a low dielectric loss and a small temperature coefficient of resonance frequency in resonators. Hereinafter, temperature coefficient of resonance frequency is referred to as ".tau.f". Especially, such materials have been now brought into practical use in communication or broadcasting equipments, such as receivers for a satellite broadcasting or automobile telephone.
BaO-TiO.sub.2 or MgO-CaO-TiO.sub.2 system dielectric ceramics have heretofore been known in such applications. However, since those heretofore known BaO-TiO.sub.2 system or MgO-CaO-TiO.sub.2 system dielectrics have a low dielectric constant of the order of 20 to 40, they are disadvantageous in that large dimensions are required because of their low dielectric constant when those dielectrics are employed as resonators at a relatively low frequency region of about 1 GHz. As represented by automobile telephones, currently, there is an increasing demand for dielectric resonators for use in frequencies near 1 GHz and, with this need, much effort or attempts have been made to provide miniaturized resonators. In order to make possible the miniaturization, dielectrics having a high dielectric constant .epsilon., a low dielectric loss and a small .tau.f are required. Particularly, in the case of a coaxial resonator used for automobile telephones, the dielectric constant of a dielectric ceramic used for the coaxial resonator should be sufficiently high without decreasing the Q of the resonator to unacceptable levels (wherein Q is the reciprocal of dielectric loss tan .delta., i.e., Q=1/tan .delta.). More specifically, an increase in the dielectric constant of the dielectric ceramic permits the reduction in the height of the resonator required for the use at the desired resonance frequencies, thereby making possible the miniaturization of the resonator. While in order to minimize deleterious effects due to harmonics, the diameter of the resonator should be reduced, the reduction in the diameter leads to an unfavorable decrease in the Q value of the resonator. Therefore, it is essential that the dielectric ceramic used for the resonator has the highest possible dielectric constant, while the Q value of the resonator has to be kept at the acceptable level from the matter of the design of a resonator, in addition to the requirement that the dielectric ceramic used in the resonator should have a small dielectric loss and a small .tau.f. Such requirements can be satisfied with dielectric materials which make it possible to obtain various desired dielectric constant values with a small dielectric loss and a small .tau.f.
Heretofore, there has been known BaO-TiO.sub.2 -R.sub.2 O.sub.3 system (wherein R.sub.2 O.sub.3 represents lanthanide oxide) as high dielectric constant materials but among them materials having a sufficiently small .tau.f exhibit a small Q of 2000 to 3000 at 3 GHz and thus result in a large dielectric loss.
Taking as a representative example a BaO-TiO.sub.2 -Sm.sub.2 O.sub.3 system (Japanese Patent Publication No. 59-37 526), materials of fully small .tau.f possess a dielectric constant .epsilon. of 75 to 92 and a Q value of 2300 to 2790 at 2.2 to 2.3 GHz ("Dielectric Properties at Microwave Frequencies of the Ceramics in BaO-Sm.sub.2 O.sub.3 -TiO.sub.2 System" presented by S. Kawashima et al., at Am. Ceram. Soc., 87th, Annual Meeting) and, particularly, at 3 GHz their Q value is reduced to an impractical level of 3000 or less and an improved Q has been required.
Therefore, an object of the present invention is to improve the foregoing disadvantages and thereby to provide improved dielectric ceramic compositions having various desired dielectric constant values in a wide range while maintaining a large Q and small .tau.f.